by Paula Hugens, eZED Ltd | 22 June 2014
For some reason underfloor insulation is treated like the poor cousin when it comes to insulating the home. The New Zealand Building Code stipulates a pitiful R 1.3 m² K/W for floor insulation; a concrete slab on grade with no insulation meets this requirement. If you have a heated floor the code minimum increases a small amount to R 1.9 m² K/W. There is no reasoning behind this but given the comments I often hear it is likely because a number of common misconceptions.
Myth 1 – Heat rises: Yes convective heat currents do rise, this is commonly known as the stack effect. Heat losses occur when a fluid carries the heat energy away, in our case for a building, the fluid is air. However in a concrete slab on grade there is no air movement so heat losses don’t occur through a convection process.
Myth 2 – More heat is lost through the roof: not necessarily, heat loss is a function of thermal resistance of the materials and the overall area of that element. In all likelihood the area of the roof is similar to the floor so the R-values of these components should be similar. Admittedly there is a small difference in the surface resistances depending on the direction of the heat flow but we are only talking about a very small 0.07 m² K/W difference here. This doesn’t explain how roof insulation minimums are from Myth 4 R2.9 to R3.3 m² K/W.
Myth 3 – Ground storage: The ground under the floor slab stays at a very steady temperature all round and is usually around 10 to 12 °C in New Zealand. This is a bit cooler than the ideal indoor temperature of 20 ºC. Heat energy always flows from warm to cold, thus as long as your house is being occupied it will be warmer than the ground and will lose energy. The ground is not storing this energy, it is gone and it’s never going to come back! You will need to chill your house well down below 10 degrees for a reasonable period to get the heat flow to change direction. This is hardly a comfortable living environment. Ground coupling as this is called, is only useful for cooling a building and it may not be desirable all year round given the cool temperate climate we have.
Myth 4 – You only need it around the perimeter: This may prevent heat losses around the perimeter of the floor where the ground temperatures are potentially much lower, but you will still be losing heat through the rest of the slab. Having explained and understood Myths 1 to 3 should make this obvious.
Myth 5 – Traditional Waffle slabs are highly insulating: Sorry, but no, waffle slabs have continuous bands of concrete ribs at regular centres that are in contact with the ground. These are thermal bridges and concrete conducts heat quite well. The perimeter is also completely exposed. Given that code minimum requires no insulation under a concrete slab a waffle pods are going to be better than nothing but not exactly energy efficient.
If you are going to ask, “Do I need a slab edge thermal break?”, the answer is well of course! Have a look at these isothermal images to give you a visual understanding of the heat losses at the slab edge.
Figure 1. Traditional Waffle Slab
Figure 2. MaxRaft© Slab
What should really concern you is that a surface temperature below 12.6 ºC will allow the formation of mould as the Relative Humidity at this point will be raised above 80%. Left unchecked this will lead to structural decay in the bottom plate. Surface condensation will occur when the temperature drops below 9.6 ºC, you don’t need surface condensation for mould to grow, just moisture laden air.
Copyright eZED Limited 2014